An all-pass network is a signal processing network that ideally passes all frequencies of circuit operation equally, but changes the phase relationship between various frequencies. It does this by varying its propagation delay with frequency. An all-pass network thus provides phase shift or phase delay without appreciably changing the magnitude characteristic of the signal. Hence, an all-pass network may also be considered a phase-shift network for operation in the radio frequency region.
Phase-shift networks having frequency cutoffs outside the bandwidth of interest are useful in high-frequency coupling circuits. Such networks may be employed in various forms of hybrid couplers or other directional coupling networks. In most such instances the phase-shift network preferably does not introduce a frequency limitation for the overall circuit. It is, of course, highly desirable that the network be low loss and be capable of compact construction. One phase-shift network which has frequently been employed, in the past, is a simple lattice network with series inductors and cross connected capacitors. Such a network provides the requisite one pole characteristic, has low loss and is economical in terms of both space and components. However, the simple lattice network may not be sufficient in terms of bandwidth because parasitic reactances become significant at high frequencies.